form_validation and regex_match?

[eluser]JackU[/eluser]
I´m using the form_validation library to validate my $_GET

I could do this:
$this->form_validation->set_rules("dir","directory","min_length[2]|regex_match[/^(\\\[a-zA-Z0-9_]+)+$/]");

But i can´t do this:
$this->form_validation->set_rules("answer","answer","min_length[2]|regex_match[/^(yes|no)$/i]");

Why?

It seems that it won´t even get sent the regex_match function in form_validation. Why is that?